Optical Systems with Holographic Gratings

Abstract

An electronic device may have a display system. The display system may include a waveguide, an input coupler, and a surface relief grating (SRG) structure. The SRG structure may be formed from a high-index material that includes titanium dioxide nanoparticles. To increase the refractive index of the high-index material, the ratio of the size of the nanoparticle core to the size of the capping layer may be increased. The high-index material may also include nanoparticles of different sizes to increase the packing density of the nanoparticles. The SRG structure may be depth modulated in a lateral direction to maximize efficiency. The SRG structure may include slanted ridges covered by an encapsulant. The SRG structure may include a blazed grating with ridges that are covered by a coating and encapsulated by an encapsulant.

What is claimed is: 
     
         1 . A display system comprising:
 a waveguide configured to propagate image light in a first direction via total internal reflection; and   a surface relief grating structure at the waveguide, wherein the surface relief grating structure comprises a plurality of ridges separated by a plurality troughs, wherein each one of the plurality of ridges has an upper surface that is separated from an upper surface of the waveguide by a distance, wherein the distance is the same magnitude for each one of the plurality of ridges, and wherein a depth of the plurality of troughs changes in the first direction.   
     
     
         2 . The display system defined in  claim 1 , wherein a width of each one of the plurality of ridges is less than 500 nanometers. 
     
     
         3 . The display system defined in  claim 1 , wherein the depth of each one of the plurality of troughs is less than 1000 nanometers. 
     
     
         4 . The display system defined in  claim 1 , wherein the plurality of ridges have a pitch and wherein the pitch is less than 1000 nanometers. 
     
     
         5 . The display system defined in  claim 1 , wherein the plurality of ridges is formed from a material having a refractive index that is greater than 1.8. 
     
     
         6 . The display system defined in  claim 1 , wherein the plurality of ridges is formed from a high-index material that includes nanoparticles and wherein each one of the nanoparticles includes a titanium dioxide core and a capping layer. 
     
     
         7 . The display system defined in  claim 6 , wherein the capping layer has a thickness that is less than 15% of the magnitude of a radius of the titanium dioxide core. 
     
     
         8 . The display system defined in  claim 6 , wherein the titanium dioxide core has a radius that is greater than 12 nanometers. 
     
     
         9 . The display system defined in  claim 6 , wherein the nanoparticles are distributed in a binder that has a refractive index that is greater than 1.55. 
     
     
         10 . The display system defined in  claim 6 , wherein the nanoparticles include a first subset of nanoparticles having a first size and a second subset of nanoparticles having a second size that is different than the first size. 
     
     
         11 . A display system comprising:
 a waveguide configured to propagate image light via total internal reflection; and   first and second surface relief grating structures at the waveguide, wherein each one of the first and second surface relief grating structures comprises:
 a blazed grating that comprises ridges formed from a first material; 
 a coating over the ridges formed from a second material that has a different refractive index than the first material; and 
 an encapsulant that conforms to the ridges and the coating, wherein the encapsulant is also formed from the first material. 
   
     
     
         12 . The display system defined in  claim 11 , wherein the first material comprises silicon nitride. 
     
     
         13 . The display system defined in  claim 11 , wherein the first material comprises silicon dioxide. 
     
     
         14 . The display system defined in  claim 11 , wherein the first material has a first refractive index, wherein the second material has a second refractive index, and wherein a difference between the first and second refractive indices is greater than 0.3. 
     
     
         15 . The display system defined in  claim 11 , wherein the ridges are wavy ridges. 
     
     
         16 . The display system defined in  claim 11 , wherein the second material comprises titanium dioxide. 
     
     
         17 . The display system defined in  claim 11 , wherein each one of the first and second surface relief grating structures further comprises:
 an anti-reflective coating formed over the encapsulant.   
     
     
         18 . The display system defined in  claim 17 , wherein the waveguide has first and second opposing sides, wherein the first surface relief grating structure is formed on the first side of the waveguide, and wherein the second surface relief grating structure is formed on the second side of the waveguide. 
     
     
         19 . The display system defined in  claim 18 , wherein the ridges of the first surface relief grating structure are at a non-zero, non-orthogonal angle relative to the ridges of the second surface relief grating structure. 
     
     
         20 . A display system comprising:
 a waveguide configured to propagate image light via total internal reflection; and   first and second surface relief grating structures at the waveguide, wherein each one of the first and second surface relief grating structures comprises:
 slanted ridges formed from a first material; and 
 an encapsulant over the slanted ridges that is formed from a second material that has a lower refractive index than the first material. 
   
     
     
         21 . The display system defined in  claim 20 , wherein the slanted ridges have sidewalls at an angle relative to an upper surface of the waveguide and wherein the angle is between 45 degrees and 70 degrees. 
     
     
         22 . A display system comprising:
 a waveguide configured to propagate image light via total internal reflection; and   a surface relief grating structure at the waveguide, wherein the surface relief grating structure includes a plurality of ridges formed from a high-index material, wherein the high-index material includes a first plurality of nanoparticles having a first size and a second plurality of nanoparticles having a second size that is different than the first size.   
     
     
         23 . The display system defined in  claim 22 , wherein each one of the first plurality of nanoparticles has a first diameter and wherein each one of the second plurality of nanoparticles has a second diameter that is less than 80% of the first diameter. 
     
     
         24 . The display system defined in  claim 22 , wherein the first and second pluralities of nanoparticles each include titanium dioxide cores.